In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
When it is desired to use electrophotographic methods to form a composite image comprising a plurality of overlapping toner images ("overlapping" meaning lying, in whole or in part, over each other), e.g., to annotate a previous image record or to form a multicolor image record such as, for example, a multicolor proof, various alternatives are available.
One such alternative is to form separate single toner images on separate transparent supports and then overlay a plurality of these separate image-bearing supports, in proper registration, to form a multiple toner image. This is an involved process requiring careful registration with previous images, and, because each successive image is physically separated from previous images by at least one support, even when virtually perfect registration has been actually achieved, the images may appear to be out of registration, depending upon the angle of viewing and other factors.
Another alternative, which avoids supports between the images, involves electrophotographically forming a toner image singly and transferring the image to a receiving element while in proper registration with toner images previously sequentially formed and transferred to the receiving element. However, such a method requires that each successive toner image be kept in proper registration with previously transferred images during its transfer from the electrophotograhic element to the receiving element. Maintaining such registration during toner transfer is an inherently slow and difficult process and is dependent upon virtually absolute dimensional stability of the electrophotographic element and the receiver element during each transfer step. It should be appreciated that it is difficult to prevent stretching, shrinkage, or other distortion of the elements while they are subjected to pressure, heat, or liquid contact during development or transfer. When such distortion occurs, registration is adversely affected.
Other methods are known, which do not require registration during toner transfer and, thus, avoid the problems inherent therein. For example, U.S. Pat. No. 3,928,033 and British Pat. No. 1,035,837 describe methods of repetitively charging, exposing, and developing electrophotographic elements to form multiple overlapping toner images thereon. Each separate image is fixed in place before each succeeding cycle is carried out, and no transfer of toner images to a separate receiver element is intended; the electrophotographic element serves as the final image-bearing element. While problems of registration during transfer are thus avoided, there are other problems associated with such methods. The photoconductive layer of elements used in such methods significantly absorb visible light (since the actinic radiation employed in each imagewise exposure in those methods is visible light), and therefore, the photoconductive layers inherently impart an overall background tint or density to the final images when viewed. This can be very undesirable for some applications, e.g., where the intention is to produce a color proof to simulate intended press print quality and to allow evaluation of the color quality of original color separation negatives. Furthermore, in the methods of those two patents imagewise exposures subsequent to the first are carried out with actinic visible light that must pass through the previously deposited toner image or images before it can reach the photoconductive layer to produce selective charge dissipation. It should be appreciated that at some point in each of those methods the imagewise visible exposing light will either be undesirably attenuated by the previously deposited toner images (which are visibly colored and thus inherently block transmission of some visible light) thus causing false latent images to be created, or, alternatively, the previously deposited toner images will not in fact have been actually representative of the hues they were intended to represent. For example, in British Pat. No. 1,035,837 the order of imaging described is to produce cyan, then magenta, then black, and, finally, yellow toner images in overlapping configuration. in order to produce the yellow image, a visible actinic light exposure is intended to pass through the previous toner images, including the black image. No matter what the visible wavelength or wavelengths of that visible actinic light are, the light will either be undesirably attenuated nonuniformly by the black toner image to cause false imaging, or the black toner will not have been a true black as intended, since an image that truly appears black must inherently absorb light significantly throughout the visible spectrum (i.e., throughout the range of wavelengths from 400 to 700 nanometers). The same sort of problem is inherent in the disclosure of U.S. Pat. No. 3,928,033, wherein the order of imaging described is to produce yellow, then magenta, then cyan, and, finally, black toner images in overlapping configuration. The patent teaches use of white light in the final exposure step involved in producing the black toner image. It should be evident that each of the previously deposited yellow, magenta, and cyan toner images will undesirably attenuate that light nonuniformly on its way to the photoconductive layer and cause some degree of false imaging.
Another method, which also forms multiple overlapping toner images directly on an electrophotographic element, but which clearly avoids the problems inherent in the methods of the U.S. and British patents just discussed, is described in allowed U.S. patent application Ser. No. 773,528, filed Sept. 6, 1985, the disclosure of which is hereby incorporated herein by reference. In the method of that application an electrophotographic element is employed, wherein the electrically conductive substrate is transparent to the actinic exposing radiation intended to be used. The method requires that, at least after one toner image is formed on the front surface of the element, all further imagewise exposures are carried out through the transparent conductive substrate (i.e., through the rear surface of the element), rather than through the toner image previously formed on the front surface. Thus, no exposure is attempted to be carried out through previously formed toner images, and the potential problems thereof are completely avoided. However, such a method does require that a high-quality conductive substrate that is transparent and non-scattering to the actinic radiation be provided, which may in some cases be difficult or inefficient to accomplish, depending, for example, on the particular actinic radiation desired to be employed. It would be desirable to avoid the need for such a substrate.
U.S. Pat. No. 4,510,223 also describes forming a plurality of toner images in overlapping configuration on an electrophotographic element. The imaging exposures are carried out with a tungsten-filament visible light source equipped with a 480 nanometer broad band filter, the visible light of which is filtered imagewise through a different separation negative for each exposure. It is stated that sufficient exposures are made through previously formed toner images that do not adversely affect the latent image desired to be produced. The reasons for this are also stated. Previous toner images are formed in layers "thin enough to have a degree of transparency" to the exposing radiation. A large degree of transparency in such toner images is not necessary, since the intention is to produce half-tone images by completely discharging the photoconductor in each area exposed. Thus, the method uses an excess of visible exposing radiation overall in order to ensure that enough visible radiation will reach the photoconductor to completely discharge the exposed areas, even though the radiation may have been significantly attenuated by previously formed toner images in some areas. The patent teaches orders of multiple imaging, wherein the first toner image formed is always a black toner image. Of course, the amount of visible radiant energy that is sufficient to punch through a partially transparent toner in some areas (e.g., a black toner) and completely discharge the photoconductor in those areas, is much more than enough to effect such complete discharge in areas having no previously formed toner. Thus, while such a method may avoid false imaging due to previous toner images, it does so by wasting energy through overexposure of untoned areas; and the method cannot be used to form continuous-tone images that depend on gradations of toner deposition height, rather than area coverage, to give visual impressions of differing degrees of visual density, because the only possible results of the method are no toner image dots (in areas of no discharge because of no exposure) or maximum density toner image dots (in areas of complete discharge because of high exposure).
It would be desirable to provide an electrophotographic method of forming a plurality of overlapping toner images, wherein imagewise exposures could be carried out through previously formed toner images without adverse attenuation of the actinic exposing radiation and without wasting energy by overexposure, and wherein the method could be used to provide continuous-tone or half-tone images, as desired. The present invention provides such a method.